Generalize pci_find_rom and use it to locate x86 video ROM in drm2.

- Make pci_find_rom take the ROM `BAR' size as a parameter, instead
  of using pci_find_mem with the ROM `BAR' to detect the size.

- Use it to find the x86 video ROM in [0xc0000, 0xe0000) in drm2,
  when nothing else reports that location.

- Adapt the one other caller in radeonfb, which already has the
  maximum ROM size handy (romsz).

XXX pullup to netbsd-7